Being submissive is wanting to or being willing to submit to orders or wishes of another. Showing an inclination in this way. An overriding desire to be pleasing. Willingly submitting without resistance to authority. Being subservient or servile.
Enjoying erotic activities in feeling powerlessness or under the control of another.
In BDSM, a submissive refers to the person who takes on the submissive role for the duration of a scene or is a submissive partner within a power exchange relationship. They are not necessarily submissive generally.
The term submissive as a noun is mostly synonymous with sub but some people use it more widely to denote anyone disposed to be submissive and would therefore say that a submissive might be a bottom, a sub or a slave. Not all slaves feel themselves to be submissive, however and not all bottoms are submissive.
A submissive person submits of their own free will and seeks to submit to another, or consents to another's dominance. Submissives vary in how seriously they take their position, training, and situation. Motivations for engaging in submissive behavior may include relief from responsibility, being the object of attention and affection, gaining a sense of security, showing off endurance, and working through issues of shame. Others simply enjoy a 'natural' feeling when they are in the presence of their more dominant partner. What are known as service-oriented submissive may also have a deep seated desire to be "of use". Submissives also vary in the extent to which they engage in play, in how often they play, and even in whether they consider their role "play" at all.
Within a S&M-only context particularly, submissive is often considered synonymous with bottom while others opine that a submissive is specifically pursuing a D/s power exchange as a key element, whereas a 'bottom' may or may not be interested (or even willing) to engage in a power exchange relationship. Some have proposed the "pitcher" and "catcher" (borrowed from baseball terminology) as more neutral terminology, with the "pitcher" delivering the sensation, the instruction, etc; and the "catcher" receiving what is "pitched."